1. Field of the Invention
The present invention relates generally to electrodes for use in batteries. More particularly, this invention relates to methods of forming charged and uncharged alloy electrodes for use in lithium (or other alkali metal) batteries.
2. Description of Related Art
In theory, some alkali metal electrodes could provide very high energy density batteries. The low equivalent weight of lithium renders it particularly attractive as a battery electrode component. Lithium provides greater energy per volume than the traditional battery standards, nickel and cadmium. Unfortunately, no rechargeable lithium metal batteries have yet succeeded in the market place.
The failure of rechargeable lithium metal batteries is largely due to cell cycling problems. On repeated charge and discharge cycles, lithium “dendrites” gradually grow out from the lithium metal electrode, through the electrolyte, and ultimately contact the positive electrode. This causes an internal short circuit in the battery, rendering the battery unusable after a relatively few cycles. While cycling, lithium electrodes may also grow “mossy” deposits which can dislodge from the negative electrode and thereby reduce the battery's capacity.
To address lithium's poor cycling behavior in liquid electrolyte systems, some researchers have proposed coating the electrolyte facing side of the lithium negative electrode with a “protective layer.” Such protective layer must conduct lithium ions, but at the same time prevent contact between the lithium electrode surface and the bulk electrolyte. Many techniques for applying protective layers have not succeeded.
Some contemplated lithium metal protective layers are formed in situ by reaction between lithium metal and compounds in the cell's electrolyte which contact the lithium. Most of these in situ films are grown by a controlled chemical reaction after the battery is assembled. Generally, such films have a porous morphology allowing some electrolyte to penetrate to the bare lithium metal surface. Thus, they fail to adequately protect the lithium electrode.
Various pre-formed lithium protective layers have been contemplated. For example, U.S. Pat. No. 5,314,765 (issued to Bates on May 24, 1994) describes an ex situ technique for fabricating a lithium electrode containing a thin layer of sputtered lithium phosphorus oxynitride (“LiPON”) or related material. LiPON is a glassy single ion conductor (conducts lithium ion) which has been studied as a potential electrolyte for solid state lithium microbatteries that are fabricated on silicon and used to power integrated circuits (See U.S. Pat. Nos. 5,597,660, 5,567,210, 5,338,625, and 5,512,147, all issued to Bates et al.).
In both the in situ and ex situ techniques for fabricating a protected lithium electrode, one must start with a smooth clean source of lithium on which to deposit the protective layer. Unfortunately, most commercially available lithium has a surface roughness that is on the same order as the thickness of the desired protective layer. In other words, the lithium surface has bumps and crevices as large as or nearly as large as the thickness of the protective layer. As a result, most contemplated deposition processes cannot form an adherent gap-free protective layer on the lithium surface.
Thus, lithium battery technology still lacks an effective mechanism for protecting lithium negative electrodes.
Additionally, one of the difficulties in using lithium as a battery electrode material is its extreme reactivity which, if not controlled, can pose a safety concern. One approach to incorporating lithium in battery cells while tempering its reactivity is lithium-ion battery technology. Lithium-ion batteries have been achieved considerable commercial success, however, these batteries have substantially less energy density than lithium metal batteries (e.g., about 2050 mAh/cc for lithium metal vs. about 570 mAh/cc for LiC6, a lithium ion electrode material).
Attempts have been made to use lithium alloy forming metals such as tin, aluminum, and antimony as negative electrodes in lithium batteries. Lithium alloys have the advantage of a volumetric energy density comparable to that of lithium metal (e.g., about 2045 mAh/cc for Li4 4Sn). However, the large volume change (e.g., expansion on the order of 200 to 300 volume %) that occurs as lithium reacts with such metals leads to crumbling of such electrodes. Consequently lithium-aluminum, lithium-tin, and related alloys have been found to not cycle reversibly in lithium batteries.
Recently, battery research groups have described the use of nanoparticles of the alloying materials in a matrix of spectator materials to avoid electrode deterioration with cycling. However, the use of passive spectator materials reduces the capacity density of the electrode materials.
The use of dense tin films on copper current collectors that do not deteriorate with cycling has also been described. Improved cyclability has been reported when tin is deposited on a copper current collector and annealed at a temperature of about 200° C. This finding provides potential battery electrode materials with increased physical integrity, however, these materials remain susceptible to deleterious interactions with battery solvent systems.
Accordingly, improved methods and structures providing battery electrode materials suitable for use in lithium (and other such reactive metal) batteries would be desirable.